linux-kernel - Re: [PATCH net-next 1/2] drivers: net: stmmac: add STMMAC_RELATIVE_FLEX

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Message-ID: <54530300-82c5-41af-adc6-8d0a45f9b37b@foss.st.com>
Date: Tue, 5 Aug 2025 13:50:19 +0200
From: Gatien CHEVALLIER <gatien.chevallier@...s.st.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
CC: Andrew Lunn <andrew+netdev@...n.ch>,
        "David S. Miller"
	<davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin
	<mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Richard Cochran <richardcochran@...il.com>, <netdev@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/2] drivers: net: stmmac: add
 STMMAC_RELATIVE_FLEX_PPS

On 8/5/25 12:30, Russell King (Oracle) wrote:
> On Thu, Jul 24, 2025 at 02:31:18PM +0200, Gatien Chevallier wrote:
>> +config STMMAC_RELATIVE_FLEX_PPS
>> +	bool "Support for STMMAC system time relative flexible PPS generation"
>> +	default n
> 
> There is no need for "default n" because the default default is n.
>

>> +	help
>> +	  Say Y to add the MAC system time to the arguments passed to the
>> +	  PTP driver when requesting a flexible PPS generation. This avoids
>> +	  the tedious task of passing an absolute time value when using sysfs
>> +	  entry.
> 
> How does a distro decide whether to enable or disable this option? What
> does it depend on?
> 
> If it's only for some platforms and not others (due to causing
> regressions) then what is a distro supposed to do with their kernels
> that support multiple platforms?
> 

Hello Russell,

Following Jakub's feedback, I sent a v2 [1] with this config removed.
The idea now is to compare the time value given to the stmmac_ptp driver
and compare it to the MAC system time. If the time value is is the past,
relative to the MAC system time, then it is considered as an offset
relative the the MAC system time.

It's not ideal as it would have probably been better to always consider
an offset but I don't know how popular and used this feature currently
is. Therefore, I did not want to break any existing script by changing
the current behavior.

I'd be very interested to have your take on this as well.

 From what I observed, it's linked to the MAC version. So enabling this
config would have meant knowing what MAC version is embedded. Supporting
multiple platforms then becomes tricky.

[1]: 
https://lore.kernel.org/all/20250729-relative_flex_pps-v2-0-3e5f03525c45@foss.st.com/